1. Field of the Invention
The present invention generally relates to an electrically powered steering device for an automotive vehicle and, more particularly, to the electrically powered steering device employing a ball screw mechanism for transmitting a drive output from an electrically driven motor to a steering shaft to selectively advance and retract the latter.
2. Description of the Prior Art
The electrically powered steering device is an instrument to assist the steering force of a steering wheel by means of an electrically driven motor and is currently available in various types. One of the types currently employed is of a design wherein a retractable steering shaft coupled with a steering mechanism for vehicle wheels is imparted an axially shifting force that is transmitted thereto from the steering wheel through a motion translating mechanism such as a rack-and-pinion mechanism for translating a rotary motion of the steering wheel into the axially shifting motion and, also, an axially shifting force that is transmitted thereto from an output of the electrically driven motor through a ball screw mechanism.
FIG. 21 illustrates an application of the conventional end-cap type ball screw mechanism to the electrically powered steering device. A rotary nut 51 has its outer periphery on which a rolling bearing assembly 53 for supporting the rotary nut 51 relative to a housing (not shown) and a rotor 54 of an electrically driven motor for driving the rotary nut 51 are mounted. A flange 52 is utilized for positioning the rolling bearing 53 and the rotor 54.
Since the rolling bearing 53 is of an inner race rotating type in which an inner race 53a is rotatable, the inner race 53a of the rolling bearing 53 is press-fitted onto the rotary nut 51 with an inner peripheral surface of the inner race 53a held in tight contact with an outer peripheral surface of the rotary nut 51. On the other hand, since the rotor 54 of the electric motor cannot be press-fitted in a manner similar to the rolling bearing 53, a portion of the outer peripheral surface of the rotary nut 51 on one side opposite to the rolling bearing 53 is formed with a knurled pattern 55 in the form of, for example, axial serrations so that when the rotor 54 is mounted on the rotary nut 51, ridges or projections of the knurled pattern 55 can be mechanically interlocked with the inner peripheral surface of the rotor 54. Thus, the rotor 54 and the rotary nut 51 are firmly coupled together so that a rotational torque can be transmitted from the rotor 54 to the rotary nut 51 without being accompanied by any relative rattling motion therebetween in a direction conforming to the direction of rotation thereof.
In the case of the structure shown in FIG. 21, an misalignment between the motor rotor 54 and the rotary nut 51 tends to occur during the assemblage as a result of failure to observe precision to such an extent as to result in increase of the rotational torque of the ball screw mechanism and/or variation in torque.
On the other hand, the ball screw mechanism is available in various types depending on the manner of circulation of balls, one of which is known as a bridge type.
FIG. 22 illustrates an example of the conventional bridge type ball screw mechanism. The ball screw shaft 71 has an outer periphery formed with an externally threaded helical groove 72 whereas the rotary nut 73 has an internally threaded helical groove 74 complemental to the externally threaded helical groove 72. A plurality of balls 75 are interposed between the externally and internally threaded helical grooves 72 and 74 so that the ball screw shaft 71 can be drivingly coupled with the rotary nut 73. A cylindrical barrel portion of the rotary nut 73 is formed with a plurality of oval bores 76 extending completely across the thickness of the wall of the cylindrical barrel portion of the rotary nut 73 while depleting respective portions of the internally threaded helical groove 74, and corresponding oval bridges 77 are engaged in those oval bores 76. Each of the bridges 77 is a component part in which a connecting groove segment 78 for communicating the neighboring turns of the internally threaded helical groove 74 together is formed. Thus, about one turn of the internally threaded helical groove 74 and the corresponding connecting groove segment 78 altogether define a ball rolling path for the balls 75. The balls 75 interposed between the externally and internally threaded helical grooves 72 and 74 within the ball rolling path can move along the externally and internally threaded helical grooves 72 and 74, then guided along the connecting groove segment 78 in the bridge 77 and return to the neighboring internally threaded helical groove 74 after having ridden over a screw thread on the ball screw shaft 71.
FIG. 23 illustrates in a development elevation the rotary nut 73, employed in the above described bridge type ball screw mechanism, as viewed from inside the rotary nut 73. In the bridge type ball screw mechanism, in the internally threaded helical groove 74, a non-circulating portion 79 that is a space where no ball exist (as shown by a cross-hatched area) is formed between the neighboring bridges 77 between turns of the ball rolling path (show by a hatched area) each corresponding to about one turn of the internally threaded helical groove 74.
During assemblage of the above described bridge type ball screw mechanism, as shown in FIG. 24, after a dummy shaft 80 in place of the ball screw shaft 71 has been set inside the rotary nut 73 so that a free end of the dummy shaft 80 is aligned with the position of the bridge 77, a number of the balls 75, for example, 17 balls corresponding to one turn of the circulating path are inserted into the rotary nut 73 and, thereafter, the balls 75 within the rotary nut 73 are successively guided to the circulating path one at a time by the use of pincette.
However, in this type of the ball screw mechanism, the balls 75 tend to enter the non-circulating portion 79 during the assemblage of the ball screw mechanism. If the ball screw mechanism is used in practice with the balls 75 mixed into the non-circulating portion 79, the balls 75 will detrimentally break the ball rolling path and there is a high risk that the ball screw mechanism would be consequently locked. In view of this, in assembling the above described bridge type ball screw mechanism, and particularly during insertion of the balls 75, the attendant worker has to take utmost care by carefully watching the work being performed so as to avoid entry of the balls 75 into the non-circulating portion 79 and/or by counting the number of the balls 75 being inserted into the circulating path. However, even though the utmost care is taken during the assemblage, no complete avoidance of the balls 75 being mixed into the non-circulating portion 79 is possible and, therefore, the possibility of the ball screw mechanism being locked cannot be avoided sufficiently.
Accordingly, the present invention has been devised to substantially eliminate the above discussed problems and is intended to provide an improved electrically powered steering device that is easy to assemble and in which any possible misalignment of the rotor of the electrically driven motor relative to the nut of the ball screw mechanism can be compensated for and, therefore, an undesirable increase of the rotational torque and variation in torque resulting from the misalignment can advantageously be eliminated.
Another important object of the present invention is to provide an improved bridge type ball screw mechanism for use in the electrically powered steering device, which is easy to assemble and in which an undesirable entry of the balls into the non-circulating portion during assemblage can be effectively and securely avoided.
In order to accomplish these objects of the present invention, a first aspect of the present invention provides an electrically powered steering device which comprises a housing, a steering shaft drivingly connected with a steering mechanism for steering wheels and extending through the housing, a motion translating mechanism for translating a rotary motion of a steering wheel into a force necessary to move the steering shaft in a direction axially thereof, a ball screw mechanism including a rotary nut and a ball screw shaft defined by a portion of the steering shaft, and an electric drive motor having a motor rotor, said motor rotor having one end portion mounted on an end portion of the rotary nut, characterized in that the rotary nut and that end portion of the motor rotor are mounted relative to each other in an adjustably alignable fashion.
According to this feature, since the motor rotor of the electrical drive motor is mounted on the rotary nut in a manner enabling an alignment therebetween to be adjusted, any possible misalignment resulting from the precision such as a radial offset between the motor rotor and the rotary nut can be compensated for by the adjustable alignment. Accordingly, an undesirable increase of and/or variation in the rotational torque resulting from the misalignment can be eliminated.
In the practice of the present invention, the motor rotor has a cylindrical inner peripheral surface and the rotary nut has a cylindrical outer surface. The cylindrical outer surface of the rotary nut may have an outer mount surface area formed with a radially outwardly extending protrusion of an arcuate longitudinal sectional shape. In this case, the radially outwardly extending protrusion are used to avoid a rotation of the rotary nut relative to the motor rotor and preferably has a multiplicity of surface indentations arranged in side-by-side fashion in a direction circumferentially of the rotary nut, whereby when the motor rotor is capped onto the rotary nut with the radially outwardly extending protrusion situated inside the motor rotor, a slight radial gap is formed between the inner peripheral surface of the motor rotor and the outer peripheral surface of the rotary nut. The radially outwardly extending protrusion may be an annular protrusion protruding radially outwardly of the rotary nut.
With the radially outwardly extending protrusion of an arcuate longitudinal sectional shape so formed that when the motor rotor is capped onto the rotary nut, the slight radial gap is formed between the inner peripheral surface of the motor rotor and the outer peripheral surface of the rotary nut, alignment between the motor rotor and the rotary nut can be adjusted about the protrusion so that any possible misalignment therebetween can be compensated for. Any possible raffling between the motor rotor and the rotary nut with respect to the direction of rotation can be avoided by the presence of the surface indentations formed on the protrusion, and therefore rotation of the motor rotor can be assuredly transmitted to the rotary nut. While both the aligning capability and the elimination of the rattling in the direction of rotation can hardly be attained simultaneously, the present invention has made it possible to attain both the aligning capability and the elimination of the raffling in the direction of rotation simultaneously because the annular protrusion is chosen to be a portion where the surface indentations for the prevention of the rattling are to be formed.
Also, in the practice of the present invention, the motor rotor has a cylindrical inner peripheral surface and the rotary nut has a cylindrical outer surface. The inner peripheral surface of the motor rotor may have an inner mount surface area formed with a radially inwardly extending protrusion of an arcuate longitudinal sectional shape. The radially inwardly extending protrusion are used for avoiding rotation of the rotary nut relative to the motor rotor, and the outer peripheral surface of the rotary nut which confronts the radially inwardly extending protrusion of the motor rotor may have a multiplicity of surface indentations arranged in side-by-side fashion in a direction circumferentially of the rotary nut, whereby when the motor rotor with the radially inwardly extending protrusion situated in the inside is capped onto the rotary nut, a slight radial gap is formed between the inner peripheral surface of the motor rotor and the outer peripheral surface of the rotary nut.
Even with this structure, alignment between the motor rotor and the rotary nut can be adjusted about the protrusion so that any possible misalignment therebetween can be compensated for. Any possible rattling between the motor rotor and the rotary nut with respect to the direction of rotation can be avoided by the presence of the surface indentations formed on the protrusion, and therefore rotation of the motor rotor can be assuredly transmitted to the rotary nut.
Where the protrusion is formed on the outer peripheral surface of the rotary nut or on the inner peripheral surface of the motor rotor as discussed above, the surface indentations may be formed by means of a form rolling technique. The use of the form rolling technique makes it easy to form the surface indentations.
Also, where the motor rotor has a cylindrical inner peripheral surface having an inner surface mount area defined therein and the rotary nut has a cylindrical outer surface having an outer surface mount area defined therein, with the outer surface mount area adapted to be covered by the inner surface mount area when the motor rotor is mounted onto that end portion of the rotary nut, at least three radial recesses of a semicircular sectional shape are preferably formed in each of the inner and outer surface mount areas in alignment with each other. In this case, a corresponding number of balls are each received in part in the respective radial recess in the inner surface mount area and in part within the mating radial recess in the outer surface mount area when the motor rotor is mounted onto that end portion of the rotary nut, so that a slight gap is formed between the inner peripheral surface of the motor rotor and the outer peripheral surface of the rotary nut. In other words, in this alternative configuration, instead of the use of the protrusion and the surface indentations, the radial recesses are utilized together with the corresponding balls.
Even this alternative configuration, the use of the plural balls and the radial recesses accommodating the respective balls makes it possible to allow the rotational torque to be transmitted from the motor rotor to the rotary nut and also to achieve an adjustable alignment between the motor rotor and the rotary nut to thereby compensate for any possible misalignment therebetween.
Where the balls are employed in combination with the radial recesses, at least one of the inner and outer mount surface areas may be formed with an axially extending insert groove communicating between each of the radial recesses and an annular end face of the rotary nut adjacent the motor rotor or an annular end face of the motor rotor adjacent the rotary nut. The respective insert groove may be formed on either the outer peripheral surface of the rotary nut or the inner peripheral surface of the motor rotor, or the both.
The formation of the insert grooves allows the balls to be smoothly guided in between the radial recesses in the motor rotor and the rotary nut.
A second aspect of the present invention provides an electrically powered steering device which includes a housing, a steering shaft drivingly connected with a steering mechanism for steering wheels and extending through the housing, a motion translating mechanism for translating a rotary motion of a steering wheel into a force necessary to move the steering shaft in a direction axially thereof, a ball screw mechanism including a rotary nut and a ball screw shaft defined by a portion of the steering shaft, and an electric drive motor having a motor rotor, said motor rotor having one end portion mounted on an end portion of the rotary nut. In this electrically powered steering device, in order to provide a slight radial gap between the inner peripheral surface of the motor rotor and the outer peripheral surface of the rotary nut, not only do the rotary nut and the motor rotor have cylindrical outer and inner peripheral surfaces, respectively, but the rotary nut has at least three circumferentially spaced recesses defined in an outer mount surface area of the outer peripheral surface thereof, and the motor rotor has radially extending throughholes defined therein so as to extend completely across a wall of the motor rotor at respective locations where, when the motor rotor is mounted onto the rotary nut, the radial throughholes align respectively with the recesses in the rotary nut. Engagement members are accommodated within the radial throughholes in the motor rotor; respectively, and plug members are externally inserted into the throughholes to clog radially outward openings of those radial throughholes to urge the respective engagement members towards the corresponding recesses in the rotary nut.
According to this structure, since the motor rotor is mounted on the rotary nut with the slight radial gap formed between the inner peripheral surface of the motor rotor and the outer peripheral surface of the rotary nut and since the engagement members are carried by the motor rotor in alignment with the radial recesses in the rotary nut, by adjusting the engagement between the engagement members with the radial recesses and also by the alignment function obtained at the points of engagement between the engagement members and the corresponding radial recesses, any possible misalignment of the motor rotor relative to the rotary nut can be compensated for. For this reason, an undesirable increase of and/or variation in the rotational torque can be eliminated advantageously. Also, since the engagement members are accommodated within the throughholes defined in the motor rotor, even though the structure is employed in which the motor rotor and the rotary nut are mounted relative to each other by means of the engagement members, it can easily be assembled.
Each of the radial recesses in the rotary nut may be of a semispherical sectional shape or flat-bottomed. Where each radial recess is of a semispherical sectional shape, each of the engagement members preferably has at least one end shaped to represent a semispherically rounded in a shape complemental to the sectional shape of the corresponding recess. On the other hand, where each radial recess is flat-bottomed, each of the engagement members preferably has a flat end face complemental to the shape of the bottom of the corresponding radial recess.
Where each radial recess is of a semispherical sectional shape and each of the engagement members preferably has at least one end shaped to represent a semispherically rounded in a shape complemental to the sectional shape of the corresponding recess, as is the case where the balls are employed, a compensating function to compensate for a possible misalignment which would occur in transmission of the rotational torque and in an alignment function can be obtained by the engagement members each having a rounded end.
Where each radial recess is flat-bottomed and, correspondingly, each of the engagement members has the flat end face, no automatic alignment function can be obtained, but during assemblage of the rotary nut and the motor rotor together, a possible misalignment between the motor rotor and the rotary nut can be eliminated by adjusting the amount of each engagement member urged into the corresponding radial recess to such an extent as to eliminate any possible rocking motions of any one of the rotary nut and the motor rotor. Also, in the case of this structure, unlike the case in which inclination is automatically aligned, it is possible to avoid occurrence of inclination of the rotary nut relative to the motor rotor.
Where each of the throughholes is to be clogged by the corresponding plug member after the associated engagement member has been inserted into the throughhole, the respective plug member preferably has an external helical thread formed on an outer periphery thereof for adjustable threading into the corresponding throughhole.
The use of the externally helically threaded plug member that can be fastened into the corresponding throughhole makes it possible to allow the plug member to be easily mounted in the throughhole and also to adjust the gripping force acting from the motor rotor to the rotary nut by way of the associated engagement member when the plug member has been fastened. For this reason, the gripping force at any point in the direction circumferentially of the rotary nut can be equalized by the adjustment discussed above.
In the structure wherein the throughholes, the engagement members and the corresponding radial recesses are employed, in place of the semispherically sectioned radial recesses, the radial recesses may have an oval shape having a long axis lying parallel to a longitudinal axis of the rotary nut, or an elliptical shape depicted by connecting two semicircles through parallel straight lines.
Where the radial recesses of the oval or elliptical shape having its long axis lying parallel to the longitudinal axis of the rotary nut are employed, an assured transmission of the rotational torque can be achieved with elimination of any rattling motion in a rotational direction while permitting a displacement in position between the throughholes in the motor rotor and the radial recesses in the rotary nut in a direction axially of the rotary nut. A compensating function to compensate for a misalignment by means of the alignment function can be obtained in a manner similar to the case in which each of the throughholes is a cylindrical hole.
In a preferred embodiment of the present invention, an elastic member may be disposed within each of the throughholes and interposed between each of the plug members and the associated engagement member. The use of the elastic member in this way is effective to render the gripping force acting on the rotary nut to be uniform.
Where the plug members are threaded into the corresponding throughholes, the interposition of the elastic member between the plug member and the engagement member within each of the throughhole is advantageous in that the gripping force can be adjusted by threading the respective plug member into the associated throughhole.
According to a third aspect of the present invention, there is provided a ball screw mechanism for the electrically powered steering device, which includes a ball screw shaft; a rotary nut having an inner peripheral surface formed with an internally threaded helical groove cooperating with the ball screw shaft to define a ball rolling guideway between the ball screw shaft and the internally threaded helical groove; a series of balls disposed in the ball rolling guideway for transmitting a force between the rotary nut and the ball screw shaft; a plurality of bridge members secured to the rotary nut and each having a connecting groove segment defined therein for communicating neighboring convolutions of the internally threaded helical groove in the rotary nut. The convolution of the internally threaded helical groove has a non-circulating portion delimited between the neighboring bridge members and where no ball move therein and in that a filler member is disposed in the non-circulating portion to fill up such non-circulating portion.
With this ball screw mechanism, since the non-circulating portion has the filler member disposed therein, any possible mixing of some of the balls into the non-circulating portions of the internally threaded helical groove during insertion of the balls successively into the ball rolling guideway can advantageously be avoided by the presence of the filler member. For this reason, there is no possibility of some of the balls intruding into the non-circulating portions as a result of an assembling error and, also, any possible locking of the ball screw mechanism which would other wise resulting from intrusion of some of the ball into non-lubricating portions can be eliminated assuredly.
In the practice of this invention, the filler member may be so shaped as to represent a shape generally similar to the non-circulating portion and is made of a separate elastic member adapted to be resiliently disposed in the non-circulating portion between the neighboring bridge members.
When the filler member is prepared from a member separate from the rotary nut, no machining to form in the rotary nut a portion corresponding to the filler member is needed and, therefore, the rotary nut may be a standard rotary nut for a ball screw mechanism having no filler member. Also, since the filler member has a shape generally similar to the shape of the non-circulating portion and is resiliently mounted in between the neighboring bridge members, mounting of the filler member onto the rotary nut can easily be achieved.
In the practice of this invention, each of the bridge member may have positioning arms engageable in the convolution of the internally threaded helical groove in the rotary nut for positioning the respective bridge member relative to the rotary nut with respect to a direction axially thereof. In this case, the arms are disposed to fill up the non-circulating portion.
The positioning arms of each bridge member are used to assuredly position the respective bridge member with respect to the axial direction. When these positioning arms are concurrently used as the filler member for filling up the non-circulating portion of the internally threaded helical groove in the rotary nut, each of the positioning arms may have a length longer than the standard one and no dedicated filler member need be disposed, thereby reducing the number of component parts and also the number of steps of assemblage.
Each of the bridge members may be inserted into a bridge receiving opening, defined in the rotary nut, from inside of the rotary nut. The structure in which each bridge member is fixed in position having been inserted into the associated bridge receiving opening from inside of the rotary nut eliminates the need to use any stop member for avoiding separation of the respective bridge member from the rotary nut and, therefore, the respective bridge member can easily be fixed in position.
Each of the bridge members may have a plurality of connecting groove segments defined therein. Formation of the plural connecting groove segments in each bridge member makes it possible to reduce the pitch between the neighboring convolutions of the internally threaded helical groove as compared with the bridge member having only one connecting groove segment. For this reason, the number of the balls that can be used can be increased to thereby increase the load capacity with no need to increase the axial length of the rotary nut.
Preferably, each of the bridge members is made of a sintered alloy. Where each bridge member is made of the sintered alloy, it can be manufactured by molding and sintering by means of an injection molding process or the like. Accordingly, neither machining nor grinding is needed, resulting in a good productivity. Thus, a less expensive manufacture is possible.
The present invention also provides an electrically powered steering device which includes a housing, a steering shaft drivingly connected with a steering mechanism for steering wheels and extending through the housing, a motion translating mechanism for translating a rotary motion of a steering wheel into a force necessary to move the steering shaft in a direction axially thereof, a ball screw mechanism including a rotary nut and a ball screw shaft defined by a portion of the steering shaft, and an electric drive motor having a motor rotor having one end portion mounted on an end portion of the rotary nut, wherein the ball screw mechanism is of a bridge type as described in any one of the above described inventions.
By this structure, the ball screw mechanism for transmitting an output from the electric drive motor to the rotary nut can be designed safe with no locking taking place in the ball screw mechanism, and, therefore, the safety factor and the reliability of the electrically powered steering device can be increased advantageously.